The correct answer to this open question is the following.
What caused West Africans to transition from controlling the exchange of goods to European slave raiders was the fact that European powers such as Great Britain, Spain, or Portugal directly competed to acquire the control of the slave trade of African people.
Everything started with John Hawkins, considered to be the first European slave trader in 1562. Years later the government of Englan granted charters to merchants and traders to open locations in West Africa where they could exploit the raw materials and natural resources such as ivory and gold. Because of this, England started to get more competition from other European nations such as Switzerland, the Dutch, and Denmark.
As for the rivers, the colonists were aware you must have water to successfully maintain life whether it be human life or plant life, it is necessary. Also being close to water helped them in the workforce, like carrying out trade or fishing It was a "win win" for them.
